1. Field of the Invention
The present invention relates to a power transmission system of a marine propulsion unit that receives an engine output, changes the speed of the output, and transmits the output to a propeller shaft. More specifically, the present invention relates to a lubricator in the power transmission system and an oil supply device arranged to supply lubricating oil to the lubricator.
2. Description of the Related Art
A conventional marine propulsion unit is disclosed in WO 2007/007707. According to WO 2007/007707, the marine propulsion unit includes a case which is disposed immediately behind a hull and supported at the rear of the hull, a propeller supported in a lower portion of the case, and a transmission unit housed in the case to receive an engine output, change the speed of the output, and transmit the output to the propeller.
In an attempt to propel a boat, if the transmission unit of the power transmission system receives the engine output, the speed of the output is changed to a desired state in the transmission unit based on a speed change operation by a crew member and is transmitted to the propeller. Then, the boat travels at a speed corresponding to the speed of the propeller.
The marine propulsion unit is provided with a lubricator for lubricating the transmission unit and with an oil supply device for supplying lubricating oil to the lubricator. In the oil supply device, an oil pump is activated by a driving force of an input-side shaft. The oil supply device controls oil from the oil pump with a control valve and supplies the oil to each clutch.
In a case where a multiplate clutch is used in the transmission, a majority of the oil, which is adhered to the clutch for lubrication, tends to flow down if the marine propulsion unit remains off for a long period of time. Accordingly, each clutch plate of the clutch is immediately lubricated at startup of the engine after a long period of being off. However, this causes a possible shortage in the lubricating oil for each of the clutch plates. To make matters worse, repetition of the above operation creates an undesirable problem on an operating life of each of the clutch plates, such as deterioration thereof at an early stage.
In addition, in a case where a plurality of planetary gears disposed one above the other in the transmission are used, the oil supplied to an upper planetary gear train flows down toward a lower planetary gear train that is located below the upper planetary gear train after lubricating the upper planetary gear train.
Therefore, part of the oil, which flows down from the upper planetary gear train, is supplied to rotating bodies of the lower planetary gear train, such as a sun gear and a ring gear, and possibly makes an unnecessary linkage rotation following these rotating bodies. This linkage rotation results in undesired resistance against the operation of the power transmission system of the marine propulsion unit, and possibly causes a power loss in the engine and a decline in fuel efficiency.
Meanwhile, in the above conventional art, an upper portion of the case of the marine propulsion unit is supported in a rear portion of the hull in a manner that a lower portion of the case can be tilted (rotated upward to the rear) along with the propeller. Then, during propulsion of the boat, the case is tilted during boat handling. However, if the case is tilted as described above, the oil tends to be concentrated in a front portion of the transmission unit, which is also tilted jointly, and become short in other portions of the transmission unit. Due to the above reason, a difficulty in stable oil suction by the oil pump may arise at tilting. In such a case, the lubrication in the transmission unit is undesirably inhibited.